1. Field of the Invention
The present invention relates generally to a vibration insulative engine mounting structure for an automotive vehicle. More specifically, the invention relates to a control system for controlling spring coefficient of an engine mount insulator for insulating vibration induced in an automotive internal combustion engine from a vehicular body so as to minimize magnitude of engine vibration to be transmitted to the vehicular body.
2. Description of the Background Art
The prior proposed control system for an engine mounting insulator for adjusting spring coefficient depending upon the vehicle driving condition has been illustrated in Japanese Utility Model First (unexamined) Publication (Jikkai) Showa 59-160416. The disclosed control system employs an engine mount insulator which defines an internal space filled with a working fluid. The fluid filled internal space is separated into two chambers. These chambers are connected with each other via a working fluid passage. A valve means is disposed within the working fluid passage in order to switch the valve position between an open position to establish fluid communication between the chambers and a closed position to block fluid communication therethrough. Therefore, according to the position of the valve means, volume of the internal space becomes different to generate different level of vibration damping force in response to the engine vibration.
In the practical control, the valve means is placed at the closed position during an engine accelerating or decelerating state to provide a higher spring coefficient and thus for a greater damping force to be generated in response to the engine vibration. On the other hand, during steady cruising or idling, the valve means is placed at the open position for increasing the volume of the chamber to provide a lower spring coefficient in order to absorb smaller magnitude and high frequency engine induced vibration.
Such prior proposed control system is successful in absorption of the engine induced vibration during engine idling state and the engine accelerating and decelerating state by absorbing smaller magnitude and high frequency vibrations and by damping greater magnitude and low frequency vibrations. However, on the other hand, in case of the vehicle cruising state, there is a possibility of causing resonation in engine due to vibration input from the vehicular body due to road shock and so forth. Particularly, in case the vibration frequency input from the vehicular body to the engine is in the vicinity of the resonance frequency of the vehicular body, resonation of the engine may cause substantial increase of magnitude of vibration to significantly degrade riding comfort of the vehicle.
For example, in case of a road having a specific road profile, the vibration magnitude becomes substantial when the undulation frequency resides close to the resonance frequency of the vehicular body at the vehicular body peak frequency while the engine resonant. On the other hand, in case of an irregularly undulated road, magnitude of vibration generated on the vehicular body becomes substantial when the undulation interval is several integral times of a wheel base dependent frequency component which is determined by a vehicular speed and wheel base length of the vehicular body.
As will be appreciated from the discussion given hereabove, the prior proposed spring coefficient control for the engine mount has not been complete in terms of cruising.